I was wrong - it's an 8320. In any case, it has T-Mobile graphics at start-up, and it works on a GSM network in Canada.
I initially thought the problem was a lack of data plan, which is why I didn't deal with it at home. But we did get AT&T's pay-as-you-go data plan, and nothing changed. I guess we'll go back to the AT&T store and double-check.
Meantime, anyone else have ideas?